Intercalation Pathway in Many-Particle LiFePO₄ Electrode Revealed by Nanoscale State-of-Charge Mapping

Abstract

The intercalation pathway of lithium iron phosphate (LFP) in the positive electrode of a lithium-ion battery was probed at the ∼40 nm length scale using oxidation-state-sensitive X-ray microscopy. Combined with morphological observations of the same exact locations using transmission electron microscopy, we quantified the local state-of-charge of approximately 450 individual LFP particles over nearly the entire thickness of the porous electrode. With the electrode charged to 50% state-of-charge in 0.5 h, we observed that the overwhelming majority of particles were either almost completely delithiated or lithiated. Specifically, only ∼2% of individual particles were at an intermediate state-of-charge. From this small fraction of particles that were actively undergoing delithiation, we conclude that the time needed to charge a particle is ∼1/50 the time needed to charge the entire particle ensemble. Surprisingly, we observed a very weak correlation between the sequence of delithiation and the particle size, contrary to the common expectation that smaller particles delithiate before larger ones. Our quantitative results unambiguously confirm the mosaic (particle-by-particle) pathway of intercalation and suggest that the rate-limiting process of charging is initiating the phase transformation by, for example, a nucleation-like event. Therefore, strategies for further enhancing the performance of LFP electrodes should not focus on increasing the phase-boundary velocity but on the rate of phase-transformation initiation.

In operando tracking phase transformation evolution of lithium iron phosphate with hard X-ray microscopy

“…Suppression of phase separation was also suggested at high-rate conditions with the simulation method 34 . A recent experimental report with an XRD technique proposed that the phase transformation pathway via a metastable crystal phase only occurs at a high charging rate condition 22 , in contrast to the other study with XANES detecting only pure phase (LiFePO 4 or FePO 4 ) with a half-charged electrode 23 . Thus, the phase transformation process in LiFePO 4 may be rate-related.…”

“…To explore the widely debated mechanism stated above, some advanced techniques (for example, in situ) have been applied in recent years to understand and examine the dynamic phase transformation in a real battery electrode as it cycles [22][23][24][25] . With a synchrotron X-ray diffraction (XRD) technique, Ogumi and colleagues 22 directly detected a metastable crystal phase of Li x FePO 4 during electrochemical phase transformation under non-equilibrium conditions (at high rate of 10C).…”

“…However, as discussed above, most conventional in situ synchrotron X-ray technologies are performed with low spatial resolution on an entire electrode. More recent ex-situ work observed the intercalation pathway in many LiFePO 4 particles (partial charged LiFePO 4 , 50% state-of-charge) with scanning transmission X-ray microscopy and found that almost all the LiFePO 4 particles were either completely lithiated or delithiated 23 . Nevertheless, the ex-situ mapping work lacks the dynamic observation and may miss some important information during the fast phase transformation in LiFePO 4 .…”

Surface reconstruction and chemical evolution of stoichiometric layered cathode materials for lithium-ion batteries

“…A recent study investigated the atomic structure of Li 2 MnO 3 after partial delithiation and re-lithiation to enable improved understanding of lithium-rich/ manganese-rich materials 27 . However, these studies were mainly performed using spatially resolved techniques, thereby inevitably lacking relevance to large-scale battery electrodes and failing to account for the inhomogeneous nature of battery electrodes [28][29][30] . In addition, the role of electrode-electrolyte interactions in the surface reconstruction and chemical evolution has been rarely studied.…”

Nanoscale visualization of redox activity at lithium-ion battery cathodes

“…However, the rational development and improvement of battery technologies requires a better view of fundamental properties of redox activity at battery electrode surfaces. Although several techniques have been developed to visualize physicochemical processes in batteries and battery materials [2][3][4][5][6][7] , mapping redox activity of battery electrodes remains challenging owing to a lack of effective analytical tools, not least because battery electrodes are typically very rough on the microscale but show activity variations on the nanoscale. To address this issue, in this paper, we describe a powerful approach for visualizing redox activity at complex composite electrodes at high spatial resolution.…”